The trend of miniaturization of solid-state electronics is likely to continue. Therefore, the ability to create complex structures at the nanoscale level will rapidly become important to the electronics industry. Traditional lithographic methods are approaching theoretical limits, and will be unable to define truly nanoscale features. Therefore, an alternative technique to simultaneously produce many identical nanoscale structures would be advantageous in terms of efficiency and cost.
Micron-sized electronic components and circuits are commonplace, and rapidly being replaced by circuitry with several hundred nanometer features. In order to make further advancements in this area, mass production techniques are needed for the assembly of 10–100 nanometer features.
Optical lithographic processes are limited in final resolution by the wavelength of the light used to expose the photoresist. Alternative techniques, such as electron beam lithography, have been developed to overcome the limitations of optical light. However, electron beam lithography is difficult to employ in a massively parallel system and, therefore, is costly.
A number of inventive approaches have been proposed to enable the production of nanoscale features on various substrates. Nanosphere lithography (NSL) is one such approach which holds promise, if certain hurdles can be overcome. In NSL, an array of ordered, nanosized particles are deposited on a substrate, forming a mask that exposes the substrate at sites between the particles. A material can then be deposited through the mask, creating a periodic structure, as proposed by U.S. Pat. No. 6,261,469, issued to Zakhidov et al.
To prepare an ordered array, particles are first dispersed in a liquid, then applied to a substrate, where the particles randomly assemble into a disordered mono or multilayer. As the liquid dries, capillary forces act upon the particles to form close packed, hexagonal arrays. Kralchevsky & Nagayama (Langmuir, Vol. 10, No. 1, 1994) studied the theoretical aspects of this method of fabrication, understanding that when the liquid film thickness approaches that of the particle size, the particles are pulled together by surface tension. The formation of an ordered monolayer was first reported by Fischer and Zingsheim in 1981, where they prepared a suspension of 312 nm particles on a glass plate (U. Fischer, H. Zingsheim, J. Vac. Sci. Technol., Vol. 19, pp. 881, 1981). After evaporation of the solvent, small portions of the plate were covered by a monolayer of spherical particles. In some areas, the spheres were close packed. Subsequent vacuum deposition through the interstices between particles in the close packed section resulted in a hexagonal pattern of triangularly shaped deposits on the substrate. The interstitial size is approximately 0.155 D, and the distance between adjacent deposits is approximately 0.8 D, where D is the diameter of the sphere.
Later, R. Micheletto (Langmuir, Vol. 11, pp. 3333, 1995) introduced a method to nucleate the hexagonal structure by tilting the substrate, which causes the organization to begin at the top of the substrate and continue downward as the drying and consolidation progresses. Dimitrov & Nagayama (Langmuir, Vol. 12, pp. 1303, 1996) modified this technique to slowly draw a plate from a bath of dispersed particles, resulting in the same effect.
However, the deficiency in all of the existing prior art is that these techniques do not form arrays with long range order. This is due to the existence of multiple nucleation sites, where the ordered arrays begin to form. When nucleation sites grow together, they must rotate or rearrange themselves if they are to mesh perfectly together, which does not happen in the techniques used in the prior art.